In the past, known types of capacitors that have been used as electric double-layer capacitors (electric double layer condensers) have included capacitors in which a separator is interposed between a pair of polar electrodes, which are then sealed with a gasket and a metal case along with electrolyte (coin or button-type capacitors); capacitors wherein an electric double-layer capacitor unit is produced by layering and winding long electrode sheets and separators, with the unit being housed in a metal case, impregnated with electrolyte, and sealed (wound-type capacitors); and capacitors in which a long electrode thin film and separator are alternately layered to form an electrode laminate, and connections are made whereby positive electrode leads are connected to the positive electrode terminals of the electrodes and negative electrode leads are connected to the negative electrode terminals, thus producing an electric double layer capacitor unit which is housed in a metal case, impregnated with electrolyte, and sealed (stacked-type).
With electric double layer capacitors that are used in situations where power is a concern, such as in electric automobiles, it is necessary to reduce internal resistance and raise electric capacity per unit surface area, thus producing high-energy density and high power density. In order to attain these requirements, investigations have been carried out concerning reducing electrode thickness and increasing the surface area of the electrode, which involves increasing the opposing surface area of the electrode. Such requirements are merely in addition to those regarding mass production of electric double layer capacitors.
In order to reduce the thickness of electrodes, thereby increasing electrode surface area, electrodes in the form of a sheet or thin film can be produced by a method such as (1) affixing a mixture in the form of a paste or ink containing electrode material to a collector by means of coating, followed by drying (solvent removal) and rolling; or (2) preparing an electrode sheet consisting of an electrode material in advance, superposing the collector on this electrode sheet surface, and then integrating the materials with a press roll. However, increasing the electrode density and capacity is difficult with the method described in (1), and the method described in (2) is generally not used.
With the manufacturing method according to (2) above, in order to reduce the internal resistance in the electric double layer capacitor, it is important to increase contact and integration between electrode material and collector. Thus, it is generally the case that the electrode material is produced by blending an adhesive with activated charcoal (carbonaceous electric double layer forming material), acetylene black, or other conductive carbon-based powder particles. FIG. 7 shows an example of this type of electrode 10″. An electrode sheet 2 constituted by carbon particles and adhesive is a porous layer having holes 3, with an uneven surface. In this case, when a meal sheet or metal foil with a smooth surface is used as the collector 1, there are point contacts at the interface between the collector 1 and electrode sheet 2, which substantially decreases the contact surface area. This causes a deterioration in the electric double layer capacitor characteristics as a result of an increase in electric resistance due to an increase in the void regions 4 (or liquid phase regions when electrolyte has been introduced) that are interposed between the collector 1 and electrode sheet 2.
Examples of polar electrodes with which these problems can be resolved include the polar electrode provided in JP (Kokai) 11-154630, wherein a porous electrode sheet (polarizable porous sheet) and collector are laminated with a conductive intermediate layer interposed therebetween, and part of this conductive intermediate layer extends into the porous region of this electrode sheet.
The polar electrode disclosed in JP (Kokai) 11-154630 has lower internal resistance than conventional electrodes, and electric double layer capacitors produced using this electrode have exceptional electric capacity and lower internal resistance relative to conventional electric double layer capacitors. This electrode also has exceptional capacity for mass production.
However, with conventional electric double layer capacitors, when moisture is adsorbed on the electrode, the adsorbed moisture undergoes electrolysis when the capacitor is used, and this phenomenon causes loss of capacitor performance.
Thus, electrodes for electric double layer capacitors are subjected to a drying treatment prior to use in capacitors, but in order to more thoroughly prevent loss of performance in electric double layer capacitors while also increasing electric double layer capacitor productivity, it is desirable to completely remove the moisture in the electrode in a short period of time. Consequently, a demand has existed for drying processes that are carried out at higher temperatures.
The present invention was developed in view of the foregoing, and it is an object thereof to provide an electrode that has high heat-resistant adhesion between the collector and polarizable porous sheet, can withstand higher drying treatment temperatures, has low internal resistance, is easy to produce continuously, and can ensure high electric capacity and low internal resistance when used in electric double layer capacitors; a method for manufacturing same; an electric double layer capacitor that employs this electrode; and a conductive adhesive that is suitable for use in the electrode.